[OpenMP][NFC] Added release note for new `deviceRTLs` and hidden helper task

[tianshilei1992]

Summary

Added release note for new deviceRTLs and hidden helper task for LLVM
12.

[jdoerfert]

LG, some suggestions for the final version.

openmp/docs/ReleaseNotes.rst

66

"implemented (via|with|using) ... intrinsics"
non-CUDA system -> CUDA-free system (maybe)
Say that if CUDA is specified or found, the behavior did not change and CUDA is used.

Make the static NVPTX runtime replacement a new bullet point please.

"A bitcode device library is"
"If the library is not found in the default path or any of the paths defined by the `LIBRARY_PATH` environment variable, an error will be raised."
"User can also specify it via" -> "Users can specify the path to the bitcode device library via"

[tianshilei1992]

comments

[tianshilei1992]

comments

openmp/docs/ReleaseNotes.rst

 ===========================
 OpenMP 12.0.0 Release Notes
 ===========================
 
 
 .. warning::
    These are in-progress notes for the upcoming LLVM 12.0.0 release.
    Release notes for previous releases can be found on
    `the Download Page <https://releases.llvm.org/download.html>`_.
-   
 
 Introduction
 ============
 
 This document contains the release notes for the OpenMP runtime, release 12.0.0.
 Here we describe the status of OpenMP, including major improvements
 from the previous release. All OpenMP releases may be downloaded
 from the `LLVM releases web site <https://llvm.org/releases/>`_.

 - Added additional environment variables to control output from the
   ``libomptarget`` runtime library. ``LIBOMPTARGET_PROFILE`` to
   generate time profile output similar to Clang's ``-ftime-trace`` option.
   ``LIBOMPTARGET_MEMORY_MANAGER_THRESHOLD`` sets the threshold size for which
   the ``libomptarget`` memory manager will handle the allocation.
   ``LIBOMPTARGET_INFO`` allows the user to request certain information from the
   ``libomptarget`` runtime using a 32-bit field. A full description of each
   environment variable is described :ref:`here <libopenmptarget_environment_vars>`.
+
+- ``target nowait`` was supported via hidden helper task, which is a task not
+  bound to any parallel region. A hidden helper team with a number of threads is
+  created when the first hidden helper task is encountered. The number of threads
+  can be configured via the environment variable
+  ``LIBOMP_NUM_HIDDEN_HELPER_THREADS``. By default it is 8. If
+  ``LIBOMP_NUM_HIDDEN_HELPER_THREADS=0``, hidden helper task is disabled and
+  falls back to a regular OpenMP task. It can also be disabled by setting the
+  environment variable ``LIBOMP_USE_HIDDEN_HELPER_TASK=OFF``.
+
+- ``deviceRTLs`` for NVPTX platform is CUDA free now. It is generally OpenMP code.
+  Target dependent parts are implemented with Clang/LLVM/NVVM intrinsics. CUDA
+  SDK is also dropped as a dependence to build the device runtime, which means
+  device runtime can also be built on a CUDA free system. However, it is
+  disabled by default. Set the CMake variable
+  ``LIBOMPTARGET_BUILD_NVPTX_BCLIB=ON`` to enable the build of NVPTX device
+  runtime on a CUDA free system. ``gcc-multilib`` and ``g++-multilib`` are
+  required. If CUDA is found, the device runtime will be built by default.
+
+  - Static NVPTX device runtime library (``libomptarget-nvptx.a``) was dropped.
+  A bitcode library is required to build an OpenMP program. If the library is
+  not found in the default path or any of the paths defined by ``LIBRARY_PATH``,
+  an error will be raised. User can also specify the path to the bitcode device
+  library via ``--libomptarget-nvptx-bc-path=``.